Valence and Coordination of Cu Ions in High Tc Superconductor BaxY1-xCuO3-y

Cu K near-edge and extended X-ray absorption fine structure (EXAFS) have been measured for 90 K superconductor Ba x Y 1- x CuO 3- y with x =0.6. The Fourier analysis of EXAFS shows that Cu ions are coordinated with basal plane oxygens having the average Cu-O distance of 1.97 Å. Determined average oxygen coordination number of c a . 4.8 suggests the presence of axial oxygens with a similar Cu-O distance. The near-edge spectra provide an evidence for the mixed valence character of Cu ions in Ba x Y 1- x CuO 3- y between predominantly divalent states and trivalent states. The strong electron-phonon interaction may be further enhanced by a charge-coupled motion of oxygens induced by the Cu valence fluctuation.